This invention relates to methods and apparatus for laser induced etching of materials, especially structures composed of layers of materials with dissimilar reactivities and etching characteristics. In particular, the invention concerns methods and apparatus for manufacturing thin-film magnetic data transfer heads.
The benefits of increased information storage capacity of a recording medium are several and include lower cost, more powerful, lighter weight computers, and the development of more powerful computer application programs. The capacity of magnetic storage media such as hard disks and magnetic tapes is not principally limited by the medium, but by the geometry of the data transfer head used to read the data from, and write the data to, the disk or tape. The bit area is determined by the physical geometry of the data transfer head, which, in turn is limited by current methods of manufacturing the transfer head.
A magnetic recording medium, such as a hard disk or tape, typically comprises a backing medium covered by a thin film of ferromagnetic material. This film can be thought of as composed of a large number of individual magnetic dipoles, present in a uniform density, and initially randomly oriented. The data transfer head writes data onto the film by creating regions of aligned dipoles.
Information is stored on the film as a pattern of aligned dipoles. To record information, the information is converted to voltage pulses which are supplied to an energizing coil of the data transfer head. These voltage pulses cause a current in the coil, and a magnetic field in a magnetic gap of the head. Because the magnetic gap is placed in close proximity to the film, the dipoles in the film are aligned in one direction or the other, depending on the polarity of the applied voltage.
The performance of the head is determined most significantly by the gap geometry and the air-bearing surface, which jointly control the dimensions of a bit in the recording medium. Under well controlled close-coupling geometry the gap dimensions are limiting and, therefore, there is intense interest in producing smaller gaps with well defined geometry. However, the air bearing is also critically important since this surface determines the height of flying magnetic head above the recording medium. Excessive height or instability in this height will degrade the device performance. Therefore evolving designs incorporate increasingly complex air foil shapes.
The wet etching and reactive ion etching techniques currently used to fabricate data transfer heads have several disadvantages that limit their ability to form the smaller geometric structures necessary for future scaled devices. Other aspects of current methods limit low-cost mass production of these devices.
Current data transfer heads are typically formed from a wafer onto which films of various materials have been deposited. The wafer is thus a multilayer structure that includes layers of ceramic, i.e., metal oxides, such as alumina; metals, such as aluminum; metal nitrides; metal carbides; hard carbon films; permalloy; cobalt alloys; and nickel phosphorous compounds. Other metal and dielectric films may also be present. The magnetic gap and air foil must be precisely defined in these multilayer structures.
In practice the combined requirements on conventional etch process and on the mask layer necessitated by conventional etches can lead to an impossible or highly restricted processes. In an extreme recent development, some manufactures have been driven to use focused ion beam (FIB) technology to define the sensor gap. In this case a submicrometer diameter beam of ions (usually Gallium ions) is focused onto the substrate and used to ion sputter instead of chemically etch the head structure. Since only nanoamps of current are possible with FIB technology, this approach is very slow (typically removing cubic micrometers or less material per second). However no mask is needed and ion sputtering has the material insensitivity required to uniformly etch all the layers of the thin-film head without undercut.
Finally, the critical geometric structures and surface finish required can currently often only be obtained by a post-etching polishing of portions of the data transfer head. The data transfer heads are handled as individual units. That is, the wafer containing layers of metal and ceramic is diced into individual die, not as a final step in the manufacturing process, but rather as an intermediate step in the process. Each die is then extensively processed individually before it is a finished data transfer head. This individual handling of die increases considerably the time and cost of manufacturing. For example, one wafer can contain hundreds, or thousands, of die heads. Manufacturing of these individual die by current methods, requires individual handling and precise three-dimensional registration of each die for all precision patterning steps. In contrast a single precision alignment for the full wafer is required if the patterning can be done before dicing. It is this same economy that is currently driving manufacturers to ever increasing wafer size for manufacturing of silicon devices.
It is known in the art to microchemically etch thin-films and substrates using a laser. As a result of the anisotropy created by the beam, mask undercutting is minimal compared to other etch techniques. Typically, a laser beam is focused onto the material to be etched in the presence of a reactive ambient gas, liquid, or film. The laser causes a chemical reaction, localized to the area of the substrate illuminated by the laser, and results in the etching of the laser-illuminated area. The chemistry involved in the process of laser-induced microchemical etching can include photolysis (e.g., linear photochemistry) or thermal processes (e.g., thermal decomposition of the substrate, or gas pyrolysis). In photolysis, the laser energy interacts significantly with the ambient by direct absorption in the gas to form reaction products that etch the substrate. In the thermal methods, the laser heats the substrate to drive a reaction between the substrate material and the ambient. Laser pyrolytic and photolytic techniques can also be used to deposit films on substrates.
The development of laser microchemical processes for the etching of materials such as silicon is described in Ehrlich et al. (D. J. Ehrlich R. M. Osgood, and T. F. Deutsch, Applied Physics Letters 38, 1018-1020, (1981) and von Gutfield and Hodgson (R. J. von Gutfield and R. T. Hodgson, Applied Physics Letters 40, 352-354, (1982), herein incorporated by reference. These processes use vapor or liquid ambients in combination with an argon ion laser to achieve fast direct write etching of silicon and/or ceramics. Yokoyama et al. (S, Yokoyama, Y. Yamakage, and M. Hirose, Applied Physics Letters, 47, 389-391, (1985) and Gee and Hargis (SPIE proceedings 459, 132-137, (1984), also incorporated herein by reference, describe extensions of this technique using excimer lasers to etch silicon dioxide in the presence of reactive vapor ambients. General reviews of laser microchemical processing can be found in Ehrlich and Tsao (D. J. Ehrlich and J. Y. Tsao, Laser Microfabrication, Pages 1-582, Academic Press, Boston (1989) and D. J. Ehrlich and J. Y. Tsao, J. Vac Sci. Technol. B-1, 969-984, (1983)). These references are likewise herein incorporated by reference.
Laser microchemical etching has not found extensive use in defining the critical geometric structures, such as the air-foil and the magnetic gap, of data transfer heads. The multilayer structure, or wafer, from which the data transfer head are fabricated contains layers of many different materials including alumina, metal nitrides, metal carbides, hard carbon coatings, permalloy, cobalt alloys, nickel phosphorous compounds, and other elemental metal films. These materials are not alike in their optical properties or thermal properties. The process parameters required to successfully etch these materials can be so different so as to make fabrication of the critical geometric structures of a data head difficult.
For example, problems often arise in laser-induced etching because of the presence of adjacent metal and metal oxide layers, especially when the metal is aluminum. Aluminum is highly reactive, but naturally forms a self-passivating oxide layer on its surface. Aluminum is actually instantaneously combustible in air; it is known as a practical, stable material because its oxide, which is formed during the instantaneous combustion, is one of the most stable and unreactive materials known. For example, when aluminum is machined on a milling machine, the freshly exposed aluminum metal undergoes spontaneous combustion or oxidation and is then converted into an impenetrable alumina or aluminum oxide layer, which quickly grows to the point where it is thick enough to extinguish further oxidation. Only a few atomic layers of the aluminum are consumed, and aluminum appears stable and passive in normal machining. However, this process of spontaneous combustion is not so benign when aluminum is laser etched. Laser etching requires a reactive, typically a halogen, ambient to convert laser-etched material into stable volatile effluent gases.
To laser etch a reactive, self-passivating metal such as aluminum, high laser powers are necessary to break through the oxide layer coating the aluminum. However, once the unoxidized metal is exposed, the laser-heated region reacts explosively with the halogen-containing ambient. The explosions crater the aluminum disastrously. As a result, critical geometric structures such as the magnetic gap cannot be successfully formed. Similar explosive effects can take place when laser etching is used on a multilayer structure containing a metal layer and a ceramic layer as, again, there are enormous differences in the laser reactivities of metals and ceramics.
One solution to this problem has been developed by Koren and co-workers (G. Koren, F. Ho, and J. J. Ritsko, Applied Physics Letters 46, 1006-1008 (1985); Applied Physics A40, 13-34), both herein incorporated by reference. Koren used a pulsed laser with a pulse duration of only twenty nanoseconds to laser etch aluminum in an ambient containing low- pressure chlorine gas. The short pulse duration limits the heating of the aluminum such that etching does occur, but the aluminum is not heated enough to create a sustained explosion of the aluminum in the halogen ambient. This techniques works, but it is extremely slow, due the short pulse duration, low pulse repetition frequency, and low halogen pressure. It is not suitable as a low cost, high production manufacturing method of forming the critical geometric structures, such as the magnetic gap and the air bearing, of magnetic data transfer heads.
There exists a need for improved techniques for efficiently forming the critical geometric structures that are part of a data transfer head. Such techniques would allow the formation of smaller geometric structures which are required to increase the data storage capacity of magnetic media at a reasonable cost.
Accordingly, an object of the invention is to provide a new technique for forming the critical geometric structures, such as the magnetic gap of data transfer heads.
Another object of the invention is to provide more economical and faster techniques for forming smaller magnetic gaps, thus reducing the cost of increased data storage capacity in magnetic media.
Another object of the present invention is to provide a method and apparatus for reducing the time and cost involved in the manufacture of data transfer heads.
Yet another object of the invention is to reduce the handling of individual die elements in the manufacture of magnetic data transfer heads.
A further object of the present invention is to reduce the process steps and number of mask layers required to manufacture a data transfer head, and to allow the use of simple, easily removable masking materials.
Yet a further object of the invention is to provide a technique for forming the precise geometric structures of data transfer heads using continuous-wave laser etching.
An additional object of the invention is to provide a technique of laser etching that allows reactive metals, and, in a multilayer structure, adjacent layers of metals and ceramics, to be microchemically laser etched without cratering.
Another object of the present invention is also to provide a laser etching technique for forming the air-bearing structure of a magnetic data transfer head.